The present invention relates to patient fluid management systems, such as hemodialysis and fluid infusion systems and methods for patient fluid management.
Patient fluid management systems typically include a fluid exchange unit, for transferring fluids between a patient and a therapeutic device, and a hydraulic circuit, for controlling the flow of a therapeutic fluid to and from the exchange unit. The fluid exchange unit can assume a variety of forms, of which the dialyzer is the most familiar. Various fluids and fluid components can be transferred across the dialyzer membrane depending, in part, on the design of the associated hydraulic circuitry. It is this hydraulic circuitry with which the present invention is most particularly concerned.
For discussion purposes, the present invention is illustrated in the context of a kidney dialysis unit, although the invention is equally useful in a variety of other applications. In a kidney dialysis unit, the patient's blood is moved through one section of the dialyzer, while a dialysate is passed through the other. The dialysate is a solvent designed to draw certain ionic materials from the blood, through the semipermeable membrane, and into the dialysate. In some systems, the hydraulic circuitry is arranged so that the volume of dialysate passed out of the dialyzer precisely matches the volume passed into the dialyzer. In such systems, no net change occurs in the patient's body fluid volume. In other systems, however, the hydraulic circuitry is arranged so that more fluid may be withdrawn from the dialyzer than is provided. In such "ultrafiltration" systems, this difference reflects a net change in the patient's body fluid volume: a quantity of the patient's body fluid is transferred from the patient, across the membrane and into the used dialysate solution. By this technique, waste water may be removed from patients whose kidney function is seriously impaired.
In some prior art systems, ultrafiltration is effected by inserting a pump in the output line of the dialyzer. One such system is shown in U.S. Pat. No. 3,598,727 to Willock. In this device, the dialysate is routed from the dialyzer to a venturi operated with running water. The water passing through the venturi draws used dialysate from the dialyzer at a rate determined by the water flow. The used dialysate and water mixture flows to a drain.
In the '727 Willock system, the amount of ultrafiltration may be calculated mathematically using the permeability of the dialyzer membrane and the pressure drops between the input and output dialysate ports and the input and output blood ports of the dialyzer. This method, however, is cumbersome. Newer systems have eliminated the need for mathematical calculations by using a volume regulated, rather than a pressure regulated approach.
One such volume regulated system, in which the amount of ultrafiltration is determined by comparing the volume of fluid removed with the volume of fluid supplied, is shown in U.S. Pat. No. 4,172,033 to Willock. In this system, the ratio of fluid volume pumped into the dialyzer to fluid volume pumped out can be adjusted by moving the pivot point about which a lever, connecting the input pump cylinder to the output pump cylinder, pivots. By this adjustment, the amount of fluid withdrawn from, or even added to, the patient can be accurately predetermined.
U.S. Pat. No. 4,267,040 to Fresenuis shows another volumetrically regulated dialysis system wherein equal amounts of dialysate are metered into and out of the dialyzer by a pair of matched pumps. Ultrafiltration may be achieved by connecting an auxiliary withdrawal device, such as a volumetric membrane pump, in parallel with the output pump, between the output of the dialyzer and an ultrafiltrate drain. The rate of operation of such auxiliary pump determines the rate of ultrafiltration.
All of the above volumetrically regulated systems suffer from a variety of drawbacks, of which cost and complexity are among the most important.
A "batch" type of ultrafiltration system is shown in U.S. Pat. No. 3,939,069 to Granger, et al. In a "batch" technique, dialysate is continuously recirculated through the dialyzer. In the Granger, et al. system, ultrafiltration is achieved by an auxiliary peristaltic pump which sucks dialysis liquid or surplus air from the top of a dialysate reservoir. The speed of the pump can be varied to change the rate of ultrafiltration.
The Granger, et al. system is undesirable because the ionic gradient across the membrane is constantly changing, making accurate dialysis difficult. The dialysate also needs to be exchanged periodically with fresh dialysate, thereby reducing the dialysis therapy efficiency.
Accordingly, a need remains for improved patient fluid management systems.